Ultrasonic sensors are put to practical use in various kinds of apparatus including, for example, an ultrasonic diagnosing apparatus for medical use and an ultrasonic flow detector for non-destructive inspection.
Though so far the mainstream of ultrasonic sensors has been those utilizing oscillation of a piezoelectric body, the development of an ultrasonic sensor of capacitance-detection type using the MEMS technology has been progressing as a result of advancements in the MEMS technology in recent years.
This ultrasonic sensor of capacitance-detection type has an oscillator formed on a semiconductor substrate with electrodes facing each other having a cavity portion therebetween. In this sensor, a membrane oscillates in the vicinity of a resonant frequency when a voltage of a direct current or an alternating current is applied and superimposed to the respective electrodes, thereby generating ultrasonic waves. By applying this principal and modifying the structure of the above described electrodes, a 1.5 dimensional array of short-axis variable focus and a two-dimensional array of real time 3D imaging compatible have been researched and developed.
A technology concerning such an ultrasonic sensor is described in U.S. Pat. No. 6,320,239 B1 (Patent Document 1) for example, and a capacitance-detection type ultrasonic transducer using a silicon substrate as the lower (bottom) electrode is disclosed therein.
In addition, there is also disclosed a capacitance-detection type ultrasonic transducer having a structure which is formed on patterned bottom electrodes (e.g., in U.S. Pat. No. 6,271,620 B1 (Patent Document 2) and “2003 IEEE ULTRASONICS SYMPOSIUM” (U.S.) 2003, pp. 577-580 (Non-Patent Document 1)).
In addition, for example, U.S. Pat. No. 6,571,445 B2 (Patent Document 3) and U.S. Pat. No. 6,562,650 B2 (Patent Document 4) disclose technologies for forming an ultrasonic transducer of capacitance-detection type on the upper layer of a signal processing circuit formed on a silicon substrate. (Also refer to Japanese Patent Application Laid-Open Publication No. 5-6849 (Patent Document 5) and Japanese Patent Application Laid-Open Publication No. 2004-071767 (Patent Document 6))